Many digital still cameras include liquid crystal display (LCD) monitors for checking images to be captured and captured images. In addition, many digital still cameras include video output terminals and can display images on, for example, external television receivers.
One type of digital still camera can capture moving images in addition to still images. In most cases, these digital still cameras capture moving images in video graphics array (VGA) size, i.e., 640 dots in width and 480 dots in height, in the NTSC format in consideration of viewing on personal computers (See Japanese Unexamined Patent Application Publication Nos. 5-122663, 8-172609, and 2001-313896, for example).
In a video camera, i.e., a television camera, a charge coupled device (CCD) image sensor captures an image and outputs image data once every field period, as shown in the upper side of FIG. 7A. The image data is then processed and output from the camera as video signals once every field period in series, as shown in the lower side of FIG. 7A.
In FIGS. 7A and 7B, numerals 1, 2, 3, . . . are serial numbers respectively assigned to certain consecutive frames. A symbol having a numeral with a suffix A indicates the odd field in a frame indicated by the numeral, and a symbol having a numeral with a suffix B indicates the even field in a frame indicated by the numeral. Solid arrows indicate image data flows of odd fields, and dotted arrows indicate image data flows of even fields. Hereinafter, the same rules are followed in other drawings.
On the other hand, a digital still camera mainly captures still images and is suitably designed for capturing still images. Thus, in a digital still camera, a CCD image sensor captures an image and outputs image data once every frame period, as shown in the upper side of FIG. 7B. The image data per frame is split into a first image data component of the odd field, as indicated by a solid arrow, and a second image data component of the even field, as indicated by a dotted arrow. These image data components are output from the camera as video signals once every field period in series, as shown in the lower side of FIG. 7B.
The above capturing and outputting techniques are also used for capturing moving images. Thus, when a digital still camera captures moving images, the motion of the images is jerky because the interval of capturing images in a digital still camera is twice that in a video camera.
FIG. 8 shows synchronizing frequencies of the NTSC format and the PAL format and the frequency ratio. Thus, when moving images are captured (or are captured, recorded, and played back) in the NTSC format, both frames and lines of image data in the NTSC format need to be converted into those in the PAL format for viewing on a PAL television receiver.
FIGS. 9 and 10 illustrate typical techniques for converting frames. The upper sides of FIGS. 9 and 10 show image data before frame conversion. This image data corresponds to, for example, image data that is output from a CCD image sensor or recorded. The lower sides of FIGS. 9 and 10 show image data in the PAL format after frame conversion. This image data corresponds to, for example, video signals output from a camera to an external television receiver or video signals supplied to a built-in LCD monitor.
In the case of FIG. 9, a first frame to a third frame in the NTSC format are respectively used for a first frame to a third frame in the PAL format. The odd field in a fourth frame in the NTSC format is used as the odd field 4A in a fourth frame in the PAL format, and the even field is decimated. A fifth frame in the NTSC format is used for the even field 4B in the fourth frame in the PAL format and the odd field 5A in a fifth frame in the PAL format.
After some fields in the NTSC format are decimated, residual fields in the NTSC format are converted into those in the PAL format so that the ratio of frame frequency of the NTSC format to that of the PAL format is eventually 1,200:1,001.
When every 1,200 frames of image data in the NTSC format are decimated to 1,001 frames, the sequence of the decimation is complicated because the location of the field to be decimated shifts as time elapses. Thus, when the decimation is carried out under the control of a central processing unit (CPU), there is a considerable software load.
Thus, the following technique shown in FIG. 10 has been conceived: Each frame of image data is retrieved from, for example, a CCD image sensor or a recording medium every 1/30 seconds, and is converted into that in the PAL format. In this case, the ratio of frame frequency of the input image data to that of the PAL format is as follows:
30 Hz:25 Hz=6:5
Accordingly, as shown in FIG. 10, image data in the PAL format can be obtained by decimating one frame in the NTSC format every six frames, thereby enabling an easy frame conversion.
However, in this frame conversion technique, information for one frame period every six frame periods in the NTSC format is dropped out. Thus, a discontinuous point occurs every five frames in the PAL format, which prevents smooth playback. When a digital still camera captures moving images, the motion of the moving images is jerky as compared with that in a video camera, as described above. Hence, when frames of these moving images are converted with the technique shown in FIG. 10, the motion of the moving images is jerkier. Moreover, in some cases, an LCD monitor of a digital still camera cannot adapt to the 60 Hz synchronizing frequency system and cannot display images.
As shown in FIG. 8, the number of horizontal lines of the NTSC format is different from that of the PAL format. When frames of the NTSC format are converted into those of the PAL format, the number of horizontal lines also needs to be converted. Thus, when image data in the NTSC format is converted into image data in the PAL format, both frame conversion and line conversion are carried out. When these conversions are not suitably carried out, the whole converted image data lags behind by several frame periods. Some portion of memory is required for these several frame periods.
The present invention is intended to solve the above problems.